This invention relates to magnetostrictive actuators, and particularly to magnetostrictive fuel injectors and electronic valve timing actuators for internal combustion engines. More particularly, this invention relates to an apparatus and method of using magnetic flux as a feedback variable to control the level of magnetostriction in a magnetostrictive actuator.
It is believed that magnetostrictive actuators typically comprise a magnetostrictive member positioned within a solenoid coil, and a prestress mechanism for loading the magnetostrictive member to the nominal prestress value required to produce maximum magnetostriction. In the case of Terfenol-D, for example, it is believed that the magnetostrictive member may be prestressed to a value of about 7.6 MPa to maximize magnetostriction. However, it is believed that prestress may or may not be necessary, depending on the particular application and type of magnetostrictive material. It is believed that the solenoid coil generates the magnetizing force necessary to cause the desired magnetostriction in the magnetostrictive member.
The term xe2x80x9cmagnetostriction,xe2x80x9d as it is used in this disclosure, means magnetic contraction, but it is generally understood to encompass the following similar effects associated with ferromagnetic materials: the Guillemin Effect, which is believed to be the tendency of a bent ferromagnetic rod to straighten in a longitudinal magnetic field; the Wiedemann Effect, which is believed to be is the twisting of a rod carrying an electric current when placed in a magnetic field; the Joule Effect, which is believed to be a gradual increasing of length of a ferromagnetic rod when subjected to a gradual increasing longitudinal magnetic field; and the Villari Effect, which is believed to be a change of magnetic induction in the presence of a longitudinal magnetic field (i.e., inverse Joule Effect). It is believed that the Villari Effect is the magnetostriction effect of greatest importance in actuator design. It is believed that typical magnetostrictive actuators utilize the Joule Effect for generating displacement and force.
It is believed that dimensional changes that occur when a ferromagnetic material is placed in a magnetic field are often considered undesirable because of the need for dimensional stability in precision electromagnetic devices. Therefore, it is believed that manufacturers of ferromagnetic alloys often formulate their alloys to exhibit very low magnetostriction effects. It is believed that ferromagnetic materials exhibit magnetic characteristics because of their ability to align magnetic domains. It is further believed that strongly magnetostrictive materials characteristically have domains that are longer in the direction of their polarization and narrower in a direction perpendicular to their polarization, thus allowing the domains to change the major dimensions of the ferromagnetic material when the domains rotate.
Alloys of Terbium (Tb), Dysprosium (Dy), and Iron (Fe) to form TbxDy1xe2x88x92xFe2 are believed to result in a magnetostrictive material in which useful strains may be attained. For example, the magnetostrictive alloy Terfenol-D (Tb0.32Dy0.68Fe1.92), is believed to be capable of approximately 10 xcexcm displacements for every 1 cm of length exposed to an approximately 500 Oersted magnetizing field. The general equation for magnetizing force, H, in Ampere-Turns per meter (1 Oersted=79.6 AT/m) is:
H=IN/L
where I=Amperes of current; N=number of turns; and L=path length.
It is believed that Terfenol-D is often referred to as a xe2x80x9csmart materialxe2x80x9d because of its ability to respond to its environment and exhibit giant magnetostrictive properties.
While the present disclosure is described primarily with reference to Terfenol-D as the magnetostrictive material, it will be appreciated by those skilled in the art that other alloys having similar magnetostrictive properties may be substituted and are included within the scope of the present disclosure.
The present invention provides a method of controlling a magnetostrictive actuator. The methol comprises energizing a coil with a current to generate magnetic flux within the coil; measuring the amount of magnetic flux generated in the coil; and applying the amount of magnetic flux generated in the coil as a feedback variable to selectively control the amount of magnetizing force applied to a magnetostrictive member located within the coil.
The present invention also provides a method of controlling a magnetostrictive actuator. The method comprises generating a magnetizing force acting on a magnetostrictive member located within a coil; measuring flux in the magnetostrictive member; and controlling the magnetizing force in response to the measuring flux.
The present invention further provides a magnetostrictive actuator. The actuator comprises a coil; a driver electrically coupled to the coil, the driver supplying current to the coil in an operating state; a magnetostrictive element magnetically coupled to the coil in the operating state; and a sensor magnetically coupled to the magnetostrictive element and electrically coupled to the driver, the sensor detecting magnetic flux in the magnetostrictive element and outputting to the driver a signal adjusting the current supplied to the coil.